The molecule with the highest number of lone pair of electrons is

To solve this, we need to determine the Lewis structure for each molecule and count the lone pairs of electrons on the central atom and the surrounding atoms.

1. Analyze the concept

A lone pair is a pair of valence electrons that is not shared with another atom in a covalent bond. To find the total number of lone pairs, we look at the valence electrons of each atom and how many are used in bonding.

2. Step-by-step breakdown

• A. H$_2$O (Water): • Oxygen (Group 6) has 6 valence electrons. It uses 2 for bonding with two Hydrogen atoms. • This leaves 4 electrons (2 lone pairs) on the Oxygen atom. • Total lone pairs = 2.

• B. NH$_3$ (Ammonia): • Nitrogen (Group 5) has 5 valence electrons. It uses 3 for bonding with three Hydrogen atoms. • This leaves 2 electrons (1 lone pair) on the Nitrogen atom. • Total lone pairs = 1.

• C. CH$_4$ (Methane): • Carbon (Group 4) has 4 valence electrons. It uses all 4 for bonding with four Hydrogen atoms. • There are no electrons left on the Carbon atom. • Total lone pairs = 0.

• D. CO$_2$ (Carbon Dioxide): • Carbon is in the center, double-bonded to two Oxygen atoms. • Carbon uses all 4 of its valence electrons for bonding (no lone pairs on Carbon). • Each Oxygen atom (Group 6) uses 2 electrons for the double bond, leaving 4 electrons (2 lone pairs) on each Oxygen atom. • Total lone pairs = 2 (on first O) + 2 (on second O) = 4.

3. Conclusion

Comparing the totals:

• H$_2$O: 2 lone pairs
• NH$_3$: 1 lone pair
• CH$_4$: 0 lone pairs
• CO$_2$: 4 lone pairs

The molecule with the highest number of lone pairs is CO$_2$.

Exam Tip

When counting lone pairs, don't just look at the central atom! Always check the surrounding atoms (like Oxygen or Halogens) as they often carry lone pairs that contribute to the total count of the molecule.